Removal of aluminum content of concentrated salt solutions by precipitation, coagulation and sedimentation

Abstract An efficient approach for aluminum removal was developed to produce high-purity brine feed for membrane cell electrolysis, the best available technique for chlorine production. The removal of aluminum from saturated (25 wt%) brine represented a knowledge gap. The pH dependence of precipitation and sedimentation steps was investigated. The Al-content could be reduced from 1.2 to 0.2 ppm at pH 11, meeting industrial requirements. A key factor supporting Al-removal is the presence of Ca and Mg contaminants in the mine brine. This effect was observed in the presence of at least 5 ppm magnesium and/or 150 ppm calcium. No reduction in aluminum content was observed in the absence of these alkaline earths. However, in model experiments at pH 11 and 60 °C, the aluminum content decreased to 0.21 ppm in the presence of 300 ppm calcium, 0.41 ppm in the presence of 20 ppm magnesium, and 0.07 ppm in the presence of both alkaline earths. Moreover, the presence of calcium and magnesium reduced the solubility of the resulting precipitate in acidic brines. Dynamic light scattering and photometric methods confirmed the precipitation and aggregation processes. Scanning electron microscopy and X-ray diffraction analysis of the precipitates further confirmed the phenomenon of co-precipitation, identifying the products as special Al–Ca and Al–Mg compounds. Zeta potential measurements showed that the charge of the microphases in the dense electrolyte solutions was almost completely shielded. The polyacrylamide flocculant even at a concentration of 0.1 ppm significantly accelerated the removal process. The aluminum content of the supernatant was reduced to 0.3 ppm in 10 min with 2 ppm flocculant, whereas 30 min were required in the absence of the flocculant. Graphical abstract